Squarylium dyes, or squaraines, which were first reported in the 1960's (A Treibs and K. Jacob, Liebigs Ann. Chem. 1966, 592, 153; H. E. Sprenger and W. Ziegenbein, Angew. Chem. Int. Ed. Engl. 1966, 5, 894), are a class of often-fluorescent dyes with peak absorption wavelength ranging from mid-visible into infrared wavelengths (for a review, see J. Fabian, Chem. Rev. 1992, 92, 1197 and cited references). Squarylium dyes have been investigated for many applications. Such applications include uses as photoconducting layers for xerography (K. Y. Law, Chem. Rev. 1993, 93, 449), light filters (R. Bertelson and R. Sallavanti, U.S. Pat. No. 5,543,086), infrared wavelength emitting fluorescent dyes (K. Y. Law, J. Phys. Chem. 1987, 91, 5184), biological probes (E. Terpetschnig, H. Szmacinski and J. R. Lakowicz, J. Fluorescence 1993, 3, 153), and nonlinear optics (C. W. Dirk et al., J. Am. Chem. Soc. 1995, 117, 2214). Squarylium dyes are generally prepared from squaric acid and a nitrogen base, often in an alcohol solvent, by azeotropic distillation of water, and collected by filtration as described in the references above.
Long-wavelength red and near infrared wavelength absorbing dyes are represented by many different types of structures, among them dithiolenes, cyanines, squaryliums, croconiums, anthraquinones, phthalocyanines, naphthalocyanines, rylenes, mono-cationic aminium salts, di-cationic diimmonium salts, inter alia (J. Fabian, Chem. Rev. 1992, 92, 1197). Some of these red and infrared wavelength absorbing dyes also show red and infrared wavelength emission.
The squarylium dye structure offers many sites for functionalization. For example in formula (I) below, X may be chosen, for example, from H, —OH, halogen, or alkyl, and the R groups on the nitrogen may simultaneously be the same or different and chosen, for example, from H, alkyl, aryl, or alkylaryl. Furthermore, the R groups may form a cyclic structure with the arene, as described in U.S. Pat. No. 5,543,086, the contents of which is expressly incorporated by reference herein in its entirety, one example of which is compound (II) below. One or more of the positions ortho- to the nitrogen atoms in formula (I) may also be substituted with heteroatoms or other groups.

It would be beneficial to provide squarylium dyes with improved design flexibility via functionalization thereof thereby yielding desirable photophysical, solubility, thermal stability, and/or light stability properties, for example. Highly soluble squarylium dyes are desirable, as the low solubility of many squarylium dyes may be a limiting factor in many applications. For example, the solubilities of squarylium dyes derived from m-dialkylaminophenols are <10 gm/L in many common solvents, and <1 gm/L in some alcohols.